Method of manufacturing hot-worked elongated products, in particular bar or pipe, from high alloy or hypereutectoidal steel

ABSTRACT

The invention relates to a process for producing hot-worked elongated products, such as bars or tubes, from high-alloy or hypereutectoid steel in which a feedstock is heated to a deformation temperature and undergoes at least one deformation step. Following the at least one deformation step, the deformed feedstock is either cooled or heated at a specific temperature to achieve a uniform temperature distribution throughout the length and thickness of the deformed feedstock. Next the deformed feedstock is reheated to a temperature below the deformation temperature. The reheated feedstock is continuously rolled in a multi-stand reducing mill to its final size and then cooled by ambient air.

FIELD OF THE INVENTION

The invention relates to a process for manufacturing hot-workedelongated products, particularly bars or pipes, from high-alloy orhypereutectoid steel.

BACKGROUND

High-alloy or hypereutectoid steels, especially anti-friction bearingsteels such as 100Cr6, form grain boundary carbides and pearliticmicrostructural components when cooled from high temperatures (1100 to1250° C.). These formations impede mechanical workability andhardenability as well as chipless deformation. A spheroidal cementitemicrostructure suitable for further processing can be achieved onlyafter long annealing processes (spheroidal cementite annealing) of 16hours or more. Much thought has been given to the question of how toshorten the duration of this soft annealing or whether the annealing canbe replace altogether.

F. Mladen and E. Hornbogen studied the influence of thermomechanicalprocessing on the mechanical properties of 100Cr6 steel (ArchivEisenhuettenwesen 49 (1978) No. 2, pp. 449 to 453). Austenitizing wascarried out above the temperature at which Fe₃ C completely dissolveswhich, given a 0.99 C w/o, is somewhat less than 1100° C. Hot rollingbegan at 1100° C. with simultaneous cooling to 720° C. Cooling from 720°C. to ambient temperature was accomplished by water quenching. Thedetails of the deformation sequence are not discussed in the article.The thermomechanically treated microstructure displayed such a finelydispersed distribution of carbides that the resolution limits of theoptical microscope were reached. The reason for this improveddistribution was the increase in dislocation density and the subgrainboundaries created by dislocations, which resulted in new nucleationsites for the carbides.

A process for producing cylindrical rolled bodies from steel 0.7 to 1.2with a carbon w/o is known from DE PS 2361330. In this process, steelwire that has been hot-rolled at 1000° C. is rapidly cooled to atemperature that corresponds to its lower pearlite range. The steel wireis then isothermally transformed and brought to a hardness of 50 HRC bycold drawing without intermediate annealing. The rapid cooling of thewire and its subsequent isothermal transformation results in amicrostructure of fine-lamellar pearlite. This enables the wire to bedrawn, after being descaled and phosphatized, without any interveningannealing.

SUMMARY OF THE INVENTION

The object of the present invention is to describe an especiallyeconomical process for producing hot-worked elongated products,especially bars or tubes, from high-alloy steel or hypereutectoid steel,especially anti-friction bearing steel, in which a microstructure isproduced that is extremely well suited, without prior soft annealing,such as to spheroidal cementite annealing, for further chiplessprocessing and final heat treatment. A further object is to describe aprocess for producing a microstructure that is also suitable, withoutprior soft annealing, for further metal-cutting processing with asubsequent final heat treatment.

The coordinated process steps of the invention make it possible toproduce the desired microstructure, whereby, in the case of theanti-friction bearing steel, a brinell hardness less than or equal to280 HB 30, preferably less than 250 HB30, is achieved. Thismicrostructure also makes it possible to feed hot-worked tubes directlyto a processing unit, without soft annealing. The manufacturing processof the present invention is especially economical, because it omits softannealing and the transport and work steps associated therewith. Thehot-worked elongated products according to the invention can beprocessed by cold drawing, cold pilger rolling, cold rolling or crossrolling.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a structure after a prior art procedure including spheroidalcementite annealing.

FIG. 2 is a structure after the procedure according to the inventionwithout an annealing step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The individual steps that contribute to the success of the processaccording to the invention are explained in what follows: The firstprocess step, which occurs after the initial deformation and beforereheating for subsequent continuous rolling, is equalizing a temperatureusing a controlled heating or cooling to achieve temperatureequalization over the length and circumference of the rolled material,which has various temperatures. The equalization temperature is lowerthan the preset temperature of the reheating furnace. The purpose ofthis measure is, first of all, to precisely adjust the temperature ofthe rolled material, and taking into account the opportunities toregulate temperature in the reheating furnace. Secondly, the measure isintended to achieve the most precise and reproducible conditionspossible for the temperature-dependent measurement of wall thicknessthat takes place before the tube enters the reducing mill. The measurechosen, (either heating or cooling), depends on the thickness of thematerial to be rolled. For example, in the case of a pipe push bencharrangement, the temperatures of thick-walled tubes after the initialdeformations of piercing, elongation and striking are above 700° C.because the large mass retains heat. In such cases, temperatureequalization is achieved by controlled cooling to a preestablishedequalization temperature in the range between 650° and 700° C. Inthin-walled tubes, which cool very quickly, temperatures are frequentlybelow 650° C. In this case, temperature equalization is achieved bycontrolled heating to a preestablished equalization temperature in theaforementioned range of 650° to 700° C.

Actual reheating is carried out either to a temperature below Ac₁(Critical temperature between pearlite phase field and austenite phasefield on heating) but above 650° C., or to a temperature above Ac₁ butbelow A_(cma) (critical temperature between cementine-austenite phasefield and austenite phase field where a=the start of the carbidedissolution region). It is necessary to take into account the well-knownfact that the Ac₁ or A_(cma) temperature depends primarily on the carboncontent of the material used and on its deformation history. The formerof the temperature ranges mentioned above corresponds to the secondphase region α+Fe₃ C in the continuous TTT diagram, while the lattertemperature range corresponds to second phase region γ+Fe₃ C.

A further measure in the proposed combination of coordinated processsteps relates to the final continuous rolling process, preferably in astretch reducing mill. Unlike other rolling methods, this rapidcontinuous rolling offers few opportunities for intervention. It isnonetheless important for the proposed process that, first of all, aminimum partial deformation, expressed as the stretching λλ≧1.03. bemaintained in the reducing mill per each stand and that, secondly, aminimum stretching degree be maintained for the total deformationλλ≧1.5. In special cases, the total stretching can even be somewhatdeeper, for instance, λλ≧1.4. In addition, any temperature increase thatoccurs during rolling due to loss work, or any temperature decrease thatresults from excessive cooling, should be minimized. In all cases,rolling must take place in the given two-phase region, and the rolledmaterial leaving the final stand must have a temperature correspondingto that of the region in question. This means that during the preferredrolling in the γγ+Fe₃ C region, the temperature of the rolled materialmust not exceed A_(cma). Compliance with this narrow temperature rangeis achieved by cool means control; additional heat, in special cases,from an external heating device; and variations in the geometry of therolls, roll speed and pass reduction. In roller geometry, the pressedlength is especially significant.

The process according to the invention is generally applicable for allknown tube-making processes that end in a reducing mill with or withoutdraught or in a sizing mill. For example, the process can be used on acontinuous tube train, a plug train or an Assel mill. In particular, itis suitable for the push bench method of producing seamless tubes ofanti-friction bearing steel. The feedstock for the process according tothe invention can be ingot cast material (forged or rolled) or strandcast material (square or round), whereby the strand cast material isdeformed and annealed in a known manner prior to rolling. Tests haveshown that the process can be used especially advantageously when thechemical analysis of the known anti-friction bearing steel is modified.This relates, firstly, to the sulphur and phosphorous content and,secondly, to the ratio of chromium to carbon. To avoid possible melt-outat the grain boundaries when deformation rates rise, the maximum sulphurand phosphorous contents should each equal 0.005 w/o, taking intoaccount the ratio of manganese to sulphur due to the suppression of FeS.The melt-out danger results from the high deformation temperaturesrequired during the initial deformation steps, when deformation ratesare such as to lead to corresponding temperature increases. For thisreason, the deformation rate in the initial deformation steps isselected in such that the temperature in the interior of the rolledmaterial, (the least advantageous point), does not exceed 1170° C. Inaddition, low S and P contents have an advantageous effect on anysubsequent chipless deformation.

With respect to secondary metallurgy, the declining S and P contents arealso advantageous in establishing a low oxygen content in the melt,which leads to an improvement of the oxidic purity.

The chromium-to-carbon ratio should be in the range of 1.35 to 1.52,preferably 1.45. The carbon content then equals 0.94 w/o, for example,while the chromium content equals roughly 1.36 w/o. Undesirable carbidebanding can be positively influenced via this ratio.

When anti-friction bearing steel is used, the cost advantage thatresults from omitting soft annealing, which otherwise would benecessary, can be further increased by using a strand cast bar with nopredeformation, (in the cast state and without prior heat treatment(diffusion)), as the feedstock.

Another improving measure relates to the cooling step that follows thefinal deformation. After leaving the rolling mill, the rolled materialis cooled in resting air or by an air shower to a temperaturecorresponding to a microstructure located above the martensite point andbelow the bainite nose in the TTT diagram. The deformed material is heldin this area isothermally for several hours. This method has provedadvantageous in the reduction of internal stresses. This step can becarried out by placing the rolled material on a cooling bed covered at asuitable point in a heat-insulating manner, or by feeding the rolledmaterial to a temperature equalization furnace or tempering furnace.

To dispense with the hardening of individual finished products aftermachining, it is further proposed that the rolled material, aftercooling, be heated to a temperature in the range 600° to 700° C., cooledand then tempered at a temperature in the range 180° to 210° C. Afterthe heating and tempering, the rolled material has a hardnesscorresponding to the required final hardness of the finished product.

The proposed new process technology for manufacturing hot-workedelongated products, especially bars or tubes, from anti-friction bearingsteel has the following advantages:

a) The process eliminates investment expenditures for a specialannealing furnace and operating costs for long-term spheroidal cementiteannealing.

b) The process eliminates transport and work steps (annealing,straightening) and thus reduces opportunities for defects, resulting, atshorter operational run times, and in more economical hot-workedproducts or cheaper feedstocks for further deformation steps.

c) The process improves material exploitation by shortening worksequences and attaining low decarburization depths due to omission ofoxidizing annealing. This leads to small allowances and thus lowermachining volumes and allows customers to retain their gripping clampdimensions.

d) The process eliminates the requirement for straightening. Due to thereduced deformation temperature, the rolled material leaving the rollingmill has greater rigidity and becomes sufficiently straight on thecooling bed. Straightening can therefore be omitted, as a rule.

e) The process produces is markedly fine grained microstructure. Duringheat treatment, this leads to higher and more homogeneous hardness andbetter toughness. This has a positive effect on the later useful life ofthe finished product, e.g., roller bearings.

f) The process achieves a microstructure that can be subjected, withoutadditional heat treatment, to a cold deformation process, e.g., colddrawing, cold pilger rolling, cold rolling or cross rolling. Afterstress-relief annealing, cold drawn tubes have the same properties ascold pilger rolled tubes.

g) The process saves money during melt production due to the reduced Sand P contents and the Cr and C contents set at the lower limits.Minimizing carbide banding and improving oxidic purity increases theuseful properties of the finished product.

The process according to the invention will be described in greaterdetail in reference to an example. A hot-worked tube with dimensions of40.9 mm in external diameter×4.8 mm in wall thickness is to be producedfrom 100Cr6 steel on a tube push bench machine. From a strand cast bar220 mm in diameter and 11,000 mm in length, feedstock ingotsapproximately 850 mm in length are cut. The feedstock ingots of 100 Cr6steel are in the cast state, i.e., they have not been heat-treated orpredeformed. The cut ingots are placed into a rotary hearth furnace andheated to approximately 1140° C. After a total heating time of 150minutes, the ingots are removed individually from the furnace and, afterpressurized water descaling, fed to a piercing press. In the piercingpress, the initial deformation into a pierced piece takes place. In thisexample, the pierced piece has the following dimensions:

    ______________________________________           Outer diameter                    223 mm           Inner diameter                    121 mm           Wall thickness                     51 mm    ______________________________________

This deformation corresponds to a cross-sectional reduction of 29.4% andstretching of λ=1.42. In this example, the deformation rate equals 0.45s⁻¹ and influences the optimal temperature window. After the piercingpress, another deformation occurs, namely elongation in a shoulder mill.This deformation produces a shell with an outer diameter of 192 mm, aninner diameter of 112 mm and a wall thickness of 40 mm. Thecross-sectional reduction is 30.7% and the stretching λ=λ1.44. Duringthis deformation, high temperatures arise on the inner surface duringrolling. Therefore, special care must be taken to ensure that thetemperature on the shell inner surface does not exceed 1170° C.Otherwise, inner surface defects must be expected due to grain boundarymelt-out. Changes in roll speed and transport angle can be used ascontrol variables. The third deformation step is striking on the pushbench. A push bench billet with an outer diameter of 122.8 mm, an innerdiameter of 112 mm and a wall thickness of 5.4 mm is produced as theselected final size. After being pushed through a number of stands, thebillet from the bar is detached in a detaching mill in the form of aninternal die. The temperature of the billet continues to drop until theextracting of the push bar and reaches, in the described case, a levelin the range of 650° to 700° C. After extraction of the push bar, thebillet plug is created. According to the invention, the billet, beforeentering the reheating device, is subjected to controlled cooling toattain a uniform temperature distribution in the range between 650° C.and 700° C. In this case, a temperature of approximately 670° C. isstriven for. The billet is held for a certain time in a heat-insulatingbuffer, so that heat can flow from the areas of the billet with a highertemperature to the areas with a lower temperature. The heat insulationensures that the total level of the billet temperature does not fallbelow the preset target value. In this example, the temperature of thereheating furnace is set such that a temperature of roughly 740° C. isachieved in the deformation material. At this temperature, the billetruns into a stretch reducing mill. This mill comprises a large number ofthree-roll stands, which are arranged offset by 120° in a roll line. Forthe selected example with the final dimensions of 40.9×4.8 mm, 29 standsare used. The partial deformation in the base stands equals across-sectional reduction of between 7.1 and 8.1%. The total deformationequals 72.7% in keeping with a stretching λλ of 3.66. The deformationconditions are selected (i.e., the pass design and roll speed are chosenand the cooling is adjusted) in such a way as to permit a slighttemperature increase to 760° C. This ensures that deformation in thestretch reducing mill takes place completely in the two phase regionγγ+Fe₃ C. After cooling, tubes of 100Cr6 steel rolled in this mannerhave a microstructure that comes near to the spheroidal cementitemicrostructure. The finely dispersed microstructure consists ofspheroidized cementite with slight pearlite residues. The brinellhardness of the tube produced in this fashion is below 250 HB30. Thedistribution of hardness values is slight. The microstructure is finerthan that achieved by standard spheroidal cementite annealing, as can beseen by comparing FIGS. 1 and 2.

The tube produced according to the invention can be further processedwithout additional heat treatment in a chipless or metal-cuttingfashion. This processing can consist, for example, of cold drawing. Byusing one of the,

deliberate temperature control before entry into the reheating furnace,

reduced reheating furnace temperature, compared to the usual method,

rolling in the two-phase region, and,

omission of spheroidal cementite annealing lasting over 16 hours,

a much thinner decarburized layer is obtained, compared to the knownprior art. The tube dimensions needed for machining can therefore bereduced. Despite stress-relief annealing after straightening, cold-drawntubes with microstructure attainable according to the invention have thesame properties as cold-pilgered tubes.

To make clear the difference between the new process technology and theknown prior art, products of the same final size (40.9 mm outerdiameter×4.8 mm wall thickness) were also rolled of 100Cr6 steelaccording to the usual method. The hardness found in these tubes equaled328 HB30 at a reheating furnace setting of 1000° C. This hardness is sohigh that spheroidal cementite annealing is required prior to furtherprocessing.

In producing thick-walled hot tubes, (for example, 60.3×8.0 mm), it isadvantageous to control cooling based on the TTT diagram such that anisothermal holding period is introduced above the martensite point, butbelow the bainite nose. The temperature range is preferably between 240°and 300° C. After a holding period of more than 3.5 hours in thistemperature range, cooling to ambient temperature can take place.

We claim:
 1. A process for producing a hot-worked elongated element fromone of a high alloy and a hypereutectoidal steel, including the stepsof:initially deforming a feed stock at a deformation temperature byfeeding said feedstock through a reducing mill at a predetermineddeformation temperature; producing a uniform temperature distributionthroughout a length and thickness of the deformed feedstock after saidstep of initially deforming by controlled heating or cooling to apreestablished temperature; reheating the deformed feedstock to atemperature within one of a first temperature range of 650 degrees C. toAc₁ and a second temperature range of Ac₁ to A_(cma) ; deforming thereheated feedstock to a final form by continuously rolling the reheatedfeedstock in a multi-stand reducing mill for a total deformation ofλ≧1.5 and an individual deformation of λ≧1.03 through each stand of themulti-stand reducing mill and maintaining a temperature of said reheatedfeedstock within a narrow range during said continuous rolling; andcooling the finally formed feed stock to ambient temperature.
 2. Theprocess of claim 1, wherein said step of producing a uniform temperatureincludes one of controlled heating and controlled cooling to apredetermined temperature within the range 650° C. to 700° C.
 3. Theprocess of claim 1, wherein the temperature Ac₁ is 710° C. and thetemperature A_(cma) is 880° C.
 4. The process of claim 1, wherein saidstep of maintaining a temperature of said reheated feedstock to within anarrow range during said continuous rolling comprises at least one ofcooling with a cooling device, adding heat using an external heatingdevice, varying roll geometry, varying roll speed, and varying theamount of reduction per each stand in the reducing mill.
 5. The processof claim 1, wherein said feedstock comprises an anti-friction bearinghypereutectoid steel having a maximum respective sulphur and phosphoruscontent of 0.005 w/o and a chromium-carbon ratio in the range of 1.35 to1.52.
 6. The process of claim 5, wherein the chromium-carbon ratio is1.45.
 7. The process of claim 1, wherein said feedstock comprises,before said step of initial deforming, a strand cast bar without anypredeformation.
 8. The process of claim 1, wherein said step ofinitially deforming includes limiting a deformation rate such that thehighest temperature at the interior of the feedstock does not exceed1170° C.
 9. The process of claim 1, further including the step ofcooling the finally rolled feedstock to a holding temperature above amartensite temperature and below a bainite nose temperature according toa TTT diagram for the feedstock material and holding said finally formedfeedstock at said holding temperature for a holding period before saidstep of cooling said finally formed feedstock to ambient temperature.10. The process of claim 1, wherein said step of cooling the finallyformed feedstock further comprises the steps of:cooling the finallyformed feedstock; heating the cooled finally formed feedstock to atemperature with the range 650° C. to 700° C.; tempering the finallyformed feedstock at a temperature in a range of 180° C. to 210° C.; andcooling said tempered final feedstock in ambient air.